This application relates to wireless communications systems, and more particularly to repeaters for use in wireless telephone systems. Still more particularly, the application relates to repeaters for use with wireless telephone systems (or similar communications systems) having plural disparate radio interfaces differing in frequency band, format, or protocols, in which the repeater operates as a bridge to allow communication between components operating on such disparate interfaces.
In 1983, conventional xe2x80x9canalogxe2x80x9d cellular telephone service became commercially available in several cities of the United States. This event marked the first time mobile telephone service was widely available to members of the general public. Although the concept of mobile telephony was not new, the systems which had been deployed previously in the United States were extremely limited in capacity. Prior to the advent of conventional cellular service, there were long waiting lists of persons desiring to subscribe to the existing systems, and service was very expensive. System capacity was so limited that subscribers desiring to make a call sometimes had to wait tens of minutes for a radio channel to become available.
xe2x80x9cCellularxe2x80x9d systems (which term is used herein to refer to a family of mobile telephone services operating in the United States in the 824-849 MHz and 869-894 MHz frequency ranges) offer many advantages over prior mobile telephone systems. In particular, factors such as the large number of allocated radio channels, short RF path lengths, low subscriber terminal transmitter power requirements, and the ability of system operators to reuse channels within the system, combine to provide vastly greater system capacity and generally higher call quality. These factors also enable the use of hand-held telephones. In addition, service is available at relatively low cost. As a result, cellular telephone service has enjoyed great success, with consumer acceptance far exceeding what was expected by its original proponents.
Recently, the United States government has allocated radio spectrum in the 900 MHz range and in the 1.8 to 2.0 GHz range for construction of new communications systems referred to as xe2x80x9cPersonal Communications Services.xe2x80x9d Systems operating in the new PCS bands at 1.8-2.0 GHz are being installed throughout the United States at a rapid rate. Although PCS service vendors may ultimately support a variety of portable and/or mobile wireless communications and data applications, PCS systems are initially being deployed and marketed primarily as wireless telephone services operating in competition with 800 MHz cellular telephone systems.
The positioning of PCS as a competitor to cellular systems presents special challenges to PCS system operators, particularly during the initial period of system deployment. Cellular systems are relatively mature, and subscribers have learned to expect service to be available nearly everywhere they go. In many areas of the country it is not economically feasible to install PCS base stations and related infrastructure equipment due to the relatively lower population densities involved. Since some PCS providers may compete directly with wireline cellular providers, it may be difficult and expensive to obtain carrier facilities from local telephone companies to connect base stations to switching offices. Nonetheless, there will be PCS subscribers who desire service in these areas on a roaming basis.
Even in regions of relatively high population density, in many locations it may not be economically feasible to install PCS infrastructure equipment of conventional architecture during the early stages of PCS system implementation when subscriber demand is low. Due to the greater path attenuation at the higher frequencies which PCS systems use (i.e., 1.9 GHz as compared to 800 MHz for conventional cellular systems), and to lower transmitter power available from PCS subscriber units, a PCS base station provides a substantially smaller coverage area (or cell size) compared to an otherwise-equivalent 800 MHz base station. Therefore, PCS base stations (or equivalent radio transmitting and receiving facilities) must be installed with much greater geographic frequency than in equivalent 800 MHz cellular systems.
At least one cellular equipment manufacturer has implemented PCS cell sites using standard 800 MHz cell site base station equipment augmented with conventional components (e.g. amplifiers and mixers) for shifting the operating frequency to the 1.9 GHz PCS band. This approach to implementing a PCS cell site may be attractive to the equipment manufacturer because only a small amount of new equipment need be developed, and even that equipment is more or less conventional. Accordingly, the manufacturer need not expend the resources that otherwise would be required to develop a PCS cell site from scratch.
However, this approach is only available if the xe2x80x9cair protocolxe2x80x9d to be used in the PCS system is identical (except for operating frequencies) to that implemented by the existing cellular base station equipment. The terms xe2x80x9cair protocolxe2x80x9d and xe2x80x9cradio interfacexe2x80x9d are used interchangeably herein to refer to the fundamental characteristics of the radio communications medium used by terminals to communicate with other terminals and may include, for example, operating frequency bands, signal modulation methods, the format for encoding voice or data traffic, formats for call set-up messages and other signaling, and other aspects of the communication protocol. In general, if the air protocol of two systems are different, the systems are fundamentally incompatible, and terminals of one system cannot directly communicate with the terminals of the other system. In the context of cellular and PCS communications systems, the air protocol is the protocol or format which a cellular or PCS base station uses to communicate with a subscriber terminal (such as a portable telephone), and are typically defined by industry or government specifications.
Several xe2x80x9cair protocolsxe2x80x9d are available for use in PCS systems; all employ digital transmission formats. Some PCS air protocols are identical (except for operating frequencies) to air protocols used in 800 MHz cellular systems. For example, the aforementioned PCS cell site equipment, and the existing 800 MHz cell site equipment upon which it was based, were both designed to implement the U.S. standard TDMA air protocol, which is used in both 800 MHz cellular and PCS systems. However, other air protocols, which traditionally have not been used in 800 MHz cellular systems, may be preferable for use in PCS systems, because they may afford improvements in channel density, audio quality, bandwidth, noise immunity, features, or other parameters. If the air protocols of the cellular base station equipment and the PCS system are not compatible, merely adding equipment to change the operating frequency of the cellular base station equipment will not allow that equipment to function as a base station in the PCS system.
Moreover, constructing a PCS base station using cellular base station components of existing design may be advantageous to an equipment manufacturer, but it does not resolve several problems faced by the PCS system operator (including those related to economic feasibility) in initially deploying a relatively large number of PCS base stations when subscriber density is low. Compared to mature cellular systems, PCS subscriber density will likely be low system-wide during an initial period after the system is constructed. Subscriber density may remain low in some areas due to lower population density or demographic factors. In addition, due to propagation factors and lower subscriber unit transmitter power, the effective communication range in a 1.9 GHz PCS system is shorter. Therefore, even if the subscriber density were comparable, a 1.9 GHz PCS system would require a larger number of lower-capacity base stations than an equivalent 800 MHz cellular system, and would require the base stations be more uniformly distributed.
Since cellular base station equipment is generally designed for high density applications, use of modified cellular equipment in low-density PCS applications may have particularly onerous cost consequences to the PCS system operator. In addition, conventional base station equipment requires facilities to carry the cell traffic to the system operator""s land-side network. These facilities are expensive to install and carry a high recurring charge. If the PCS operator does not operate a preexisting system (such as an 800 MHz cellular system) in the geographical area, the operator will have to invest as much in PCS cell sites as the 800 MHz cellular competitors, which already have established service and a customer base in the area.
However, it is expected that a number of PCS system operators may already operate an 800 MHz cellular system in the same market. If the PCS system operator installs complete PCS base stations the operator will essentially duplicate the investment already made in cellular base stations. Although in some cases the PCS base station may share a site (including power, environmental control, and back-haul transmission facilities) with the operator""s 800 MHz PS cellular base station, the shorter range of the PCS system requires that some PCS base stations be installed at sites where no preexisting cellular base station exists. In addition, high-density cellular base stations are typically upgraded in large increments, and therefore, the system operator may already have excess capacity in 800 MHz base stations. Thus, it would be highly advantageous to PCS system operators if they were able to serve 1.9 GHz PCS subscribers using their existing investment in 800 MHz base stations and supporting infrastructure.
The problem of providing initial PCS service, at a time or in a geographical region in which subscriber demand is low, and where a preexisting, highly developed cellular system is available, may be seen as a special case of a more general problem: that of allowing subscribers of a first communications system which employs a first radio interface to access a second communications system which employs a second radio interface which is incompatible because of differences between the two in an operating characteristic. Several different trunked radio systems are commercially available, but because these systems employ different radio interfaces (or air protocols), equipment designed for different systems will not interoperate.
It is therefore an object of the present invention to provide a system for enabling terminals of a first communication system employing a first radio interface to communicate with terminals of a second communication system employing a second, incompatible, radio interface.
It is another object of the present invention to provide a system for enabling subscriber units of a first communication system employing a first radio interface to communicate with base stations of a second communication system employing a second, incompatible, radio interface.
It is a further object of the invention to provide a system for enabling a PCS mobile or portable unit to access a base station of a cellular communications system.
It is another object of the invention to provide a bidirectional repeater having a first terminal for communicating with components of a first communications system and a second terminal for communicating with components of a second communications system, thereby enabling communications between components of the first and second communications, wherein the components of the first and second communications systems are otherwise incompatible.
It is a further object of the invention to provide a bidirectional repeater having a terminal for communicating with components of a cellular communications system and a second terminal for communicating with components of PCS communications system, thereby enabling transparent communication between components of the cellular and PCS systems.
According to the present invention, a repeater is provided to allow communication between terminals, subscriber units, or other components of two or more communications systems having non-identical radio interfaces.
According to a first aspect of the invention, a first embodiment is provided in the form of a frequency translating repeater which is adapted for use in applications in which the first and second communications systems employ compatible air protocols but operate at different frequencies. The first embodiment may be used, for example, to permit subscriber terminals (i.e., portable telephones) designed to operate in a 1.9 GHz TDMA PCS system to communicate with base stations of an 800 MHz TDMA cellular system. The frequency translating repeater (also called an xe2x80x9cup-bandingxe2x80x9d repeater) comprises first means for translating the frequency of forward-signal-path signals, second means for translating the frequency of reverse-signal-path signals, means for measuring the signal strength of received signals and responsively controlling signal path gain to optimum levels, means for extracting timing information from signals on at least one of the signal paths, and means for responsively gating the output of at least one signal path of the repeater such that the repeater only transmits when required
In the aforementioned exemplary application, the first translation means receives signals transmitted by the base station in the 800 MHz band and linearly translates the signals to the 1.9 GHz band for retransmission to 1.9 GHz subscriber units (xe2x80x9cmobilesxe2x80x9d). The second translation means receives signals transmitted by mobiles in the 1.9 GHz band and linearly translates the signals to the 800 MHz band for retransmission to the 800 MHz base station. The repeater measures the received signal strength on each path and adjusts the path gain as needed to provide optimum signal levels at the base station and the mobile. In a TDMA application, the forward signal path is demodulated to recover the time slot timing transmitted by the base station. The repeater uses the time slot timing (adjusted as required for repeater-to-mobile propagation delay) to gate the reverse signal path, such that the path is enabled only during time slots occupied by a mobile operating through the repeater. This gating minimizes interference to communications which are not operating through the repeater. Since the TDMA air protocol used in 800 MHz cellular systems is compatible with that used in 1.9 GHz PCS systems (except for operating frequency), signals from the 800 MHz base station, once translated upward in frequency by the repeater, are compatible with the 1.9 GHz PCS mobiles. Similarly, signals from the 1.9 GHz PCS mobiles, once translated downward in frequency by the repeater, are compatible with the 800 MHz base station.
According to a second aspect of the invention, a second embodiment is provided in the form of a protocol converting repeater which is adapted for use in applications in which the first and second communications systems employ substantially incompatible air protocols. The repeater typically also provides frequency conversion between the operating frequencies of the two systems. The second embodiment may be used, for example, to permit subscriber terminals (i.e., portable telephones) designed to operate in a 1.9 GHz GSM PCS system to communicate with base stations of an 800 MHz TDMA cellular system. The GSM and TDMA air protocols are substantially different. As another example, the second embodiment could be used to permit subscriber terminals designed to operate in a 1.9 GHz TDMA PCS system to communicate with base stations of an 800 MHz xe2x80x9canalogxe2x80x9d cellular system. The TDMA and xe2x80x9canalogxe2x80x9d cellular air protocols are also substantially different; although signaling formats may be similar, they are not necessarily identical, and the differing formats of voice information between the two protocols makes format conversion necessary.
The protocol conversion repeater comprises a first means for communicating with a first communications system employing a first air protocol and for emulating a terminal of the first communications system, a second means for communicating with a second communications system employing a second air protocol and for emulating a terminal of the second communications system, means for transferring message traffic between the first and second communication and emulation means, and control means for supervising the operations of the communication and emulation means and for transferring, responsive to the respective air protocols of each communications system, information required to establish a message traffic connection between the first and second communications systems.
In the aforementioned exemplary application, the first communications and emulation means communicates with an existing 800 MHz cellular base station, faithfully emulating the functions and behavior of an 800 MHz TDMA cellular subscriber terminal (xe2x80x9cmobilexe2x80x9d). The second communications and emulation means communicates with a 1.9 GHz GSM PCS subscriber terminal, faithfully emulating the functions and behavior of a 1.9 GHz GSM PCS base station.
The first and second communications means are suitably interconnected to transfer message traffic (e.g., voice telephone signals) and signalling or control traffic (e.g., call set-up and supervisory messages) between the two communications systems. In general, this requires that both message traffic and signaling traffic originating in each communications system be received, demodulated, reformatted as required for the target system, remodulated, and retransmitted in the opposite system. For example, because the TDMA and GSM voice encoding formats differ, a conversion of the voice signal information across the formats is required. This may be accomplished by causing each of the first and second communications means to convert the message traffic (i.e., voice telephone signals) to a common format, and then to reconvert the message traffic to the format of the respective target system.
The control and signalling information required by various air protocols differ substantially. In addition, the protocols are subject to severe timing and other constraints. Accordingly, in general, the control information sent by a terminal in one system typically cannot be simply passed along to a terminal in the other system. The control means supervises each of the communications and emulation means and converts or modifies as necessary the control or signalling information received from one system will have the appropriate effect in the other system. In particular, the control means and the communications and emulation means cooperate as needed to establish, maintain, and terminate voice traffic connections between the 800 MHz TDMA cellular base station and the 1.9 GHz GSM PCS mobiles. For example, if the protocol conversion repeater receives from the 800 MHz TDMA cellular system a page intended for a mobile operating in the 1.9 GHz GSM PCS system, the control means causes the second communications and emulation means to format and transmit a suitable GSM page message for reception by the 1.9 GHz GSM mobiles. Other call processing messages would be similarly converted.
The inventive repeater may be co-located with a base station of the donor system (such as an 800 MHz cellular base station). However, because the range of PCS systems is often shorter than that of 800 MHz cellular systems, and therefore more PCS cells are required, the repeater will normally be in a location which is remote from the donor base station and in which PCS coverage is desired.